Airstrikes have killed at least 250 Islamic State fighters in Iraq, both Iraqi and U.S. officials reported Thursday.

The airstrikes hit around 40 Islamic State vehicles just south of Fallujah, a former Islamic State stronghold liberated by U.S.-led Iraqi coalitions last week.

These airstrikes mark the deadliest attack against the jihadist group, though officials warn their morale does not seem to be damaged, despite territorial losses.

Syrian forces have also made strides this week in regaining control of Islamic State held towns near Iraq which have effectively erased the border for jihadist fighters, making transportation and coordination easier in what Islamic State has declared its "caliphate" of the two countries.

After succeeding in their month-long coalition to seize control of Fallujah, Iraqi forces will work to retake Mosul, Iraq's second-largest city, from jihadist control.

For several weeks, U.S. intelligence officials have described the IS group as being "at its weakest point since its rapid expansion."

But Iraqi Kurdish military commanders warn that Mosul will be a much tougher fight that will require a political-military agreement between all the parties involved.